1. Field of the Invention
The invention is related to surface acoustic wave convolvers and in particular to a time multiplexed convolver for M-ARY orthogonal keyed decoding.
2. Prior Art
M-ary Orthogonal Keying data modulation is a technique used in spread spectrum communications where one basic spreading function (SF) such as a pseudo random noise (PN) sequence is phase modulated on a carrier. The basic spreading function is modulated by certain other functions, such as Walsh functions, to cause a modulation orthogonal to the basic spreading function and every other function used. The resultant response of a signal in a channel, match filtered to an orthogonally modulated signal, is zero while the matched response is a maximum.
This modulation technique is used extensively in secure communications, both for continuous communications and short message formats. Spreading the small amount of information pseudo randomly over a wide bandwidth allows for a low probability of intercept since the signal power density can be below the noise power density. Only by knowing the spreading function can the signal energy be integrated above the noise.
Conventionally, orthogonal-keyed data is recovered by implementing parallel decoding devices such as correlators or convolvers and then comparing the filtered amplitudes as in a Mark and Space receiver. This is sometimes implemented by parallel processing the orthogonal modulations using partial sums from a single decoder of the basic pseudo-random noise (PN) spreading function. Convolvers are generally used for maximum bandwidth modulation where the spreading function is changing after short intervals. However, convolvers are capable of much wider bandwidths than can be obtained in base band processing. Therefore, convolvers have a lot of excess bandwidth over that required for modulation.
This invention takes advantage of the excess bandwidth of the convolver by time multiplexing more than one reference signal and applying the time multiplexed reference signal to the signal via the reference input of the convolver. To prevent the summing bus of the convolver from simultaneously containing the product of the input signal with all of the time multiplexed reference signals, the resultant signal from the input signal and one of the time multiplexed reference signals is kept separated from the other resultant signals by segmenting the summing bus of the convolver.